The present invention relates generally to printers, and more particularly to a method for determining the skew of a printhead of a printer.
Printers include those printers having a printhead which can print a vertical line along substantially the height of the printhead. For example, a typical ink jet printer has an ink jet printhead with a vertical array (or two or more vertical arrays) of ink jet nozzles which can print a vertical line along the height of the vertical array of ink jet nozzles. With the increasing size of printheads, skew becomes a significant source of ink dot misplacement. Skew (i.e., the offset from true vertical) occurs when the printhead is not perpendicular to the direction of printhead carrier travel. There are three main sources for skew error: if the printhead die is not correctly centered on the ink bottle; if the printhead carrier is angled as it is pulled from side to side during printing; and if paper movement is not perpendicular to the direction of printhead carrier travel. The effect of skew is that features in a print swath are misaligned from true vertical and that features in a subsequent print swath do not line up with the features printed on a prior print swath. For example, when printing a xe2x80x9cverticalxe2x80x9d line, the bottom of a xe2x80x9cverticalxe2x80x9d line segment in the first swath is not centered on the top of the xe2x80x9cverticalxe2x80x9d line segment in the subsequent print swath when skew is present.
Conventional methods for determining the skew of a printhead include a method requiring printing a first single-width xe2x80x9cverticalxe2x80x9d line, then partially advancing the paper, and then printing a second single-width xe2x80x9cverticalxe2x80x9d line to partially overlap the first line. The separation distance between the overlapped portions of the two xe2x80x9cverticalxe2x80x9d lines indicates the presence of skew. There will be no separation distance if there is no skew. In this method, a counter timer analyzes the output from a sensor to determine the amount of time it took for the sensor to cross from the lead edge of the first line to the trail edge of the second line. A central processing unit determines the distance between the lines from the elapsed time and from the known speed of the moving sensor.
What is needed is an improved method for determining the skew of a printhead of a printer.
A first method of the invention is for determining the skew of a printhead of a printer. The first method includes steps a) through e). Step a) includes printing first, second and third spaced-apart images at least partially aligned substantially along the printhead scan direction. The first and third images are printed by one of an upper portion and a lower portion of the printhead, and the second image is printed by the other of the upper portion and the lower portion of the printhead between the first and third images. In the absence of printhead skew the center of the second image is equidistant along the printhead scan direction from the centers of the first and third images. Step b) includes moving a sensor in the printhead scan direction over the first, second and third images. Step c) includes obtaining sampled data points from the sensor, wherein each sampled data point indicates the presence or absence of an image. Step d) includes calculating using the sampled data points the first, second and third centers of the respective first, second and third images. Step e) includes calculating a function of the first, second and third centers, wherein the calculated function indicates the skew of the printhead.
A second method of the invention is for determining the skew of a printhead of a printer. The second method includes steps a) through e). Step a) includes printing a first plurality of spaced-apart images and a second plurality of spaced-apart images. The images of the first and second pluralities are at least partially aligned substantially along the printhead scan direction. The images of the second plurality are spaced apart from and interleaved with the images of the first plurality. The images of the first plurality are printed by one of an upper portion and a lower portion of the printhead, and the images of the second plurality are printed by the other of the upper portion and the lower portion of the printhead. In the absence of printhead skew the center of each image of the second plurality is equidistant along the printhead scan direction from the centers of the adjacent images of the first plurality. Step b) includes moving a sensor in the printhead scan direction over the images of the first and second pluralities. Step c) includes obtaining sampled data points from the sensor, wherein each sampled data point indicates the presence or absence of an image. Step d) includes calculating using the sampled data points the centers of the images of the first and second pluralities. Step e) includes calculating a function of the centers of the images of the first and second pluralities, wherein the calculated function indicates the skew of the printhead.
A third method of the invention is for determining the skew of a printhead of a printer. The third method includes steps a) through c). Step a) includes printing first, second and third spaced-apart images at least partially aligned substantially along the printhead scan direction. The first and third images are printed by one of an upper portion and a lower portion of the printhead, and the second image is printed by the other of the upper portion and the lower portion of the printhead between the first and third images. In the absence of printhead skew the distance from the second image to the first image is equal to the distance from the second image to the third image. Step b) includes moving a sensor in the printhead scan direction over the first, second and third images. Step c) includes calculating a function of the output of the sensor, wherein the calculated function indicates the skew of the printhead.
A fourth method of the invention is for determining the skew of a printhead of a printer. The fourth method includes steps a) through c). Step a) includes printing a first plurality of spaced-apart images and a second plurality of spaced-apart images. The images of the first and second pluralities are at least partially aligned substantially along the printhead scan direction. The images of the second plurality are spaced apart from and interleaved with the images of the first plurality. The images of the first plurality are printed by one of an upper portion and a lower portion of the printhead, and the images of the second plurality are printed by the other of the upper portion and the lower portion of the printhead. In the absence of printhead skew the distance from each image of the second plurality is equidistant along the printhead scan direction from the adjacent images of the first plurality. Step b) includes moving a sensor in the printhead scan direction over the images of the first and second pluralities. Step c) includes calculating a function of the output of the sensor, wherein the calculated function indicates the skew of the printhead.
Several benefits and advantages are derived from one or more of the four methods of invention. By printing images, such as rectangular blocks, the block size can be chosen to fill the field of view of the sensor which will improve the signal-to-noise level of the sensor and which, in one example, should improve resolution from generally {fraction (1/300)}-inch (if a single pixel width image is used) to generally {fraction (1/1200)}-inch (if a rectangular block which fills the field of view of the sensor is used). By printing many images along the printhead scan direction, errors in calculating the function for the images will tend to average out because variations in the velocity of the sensor along an entire sensor scan will tend to average out.